Digital lighting technologies, i.e. illumination based on semiconductor light sources, such as light-emitting diodes (LEDs), offer a viable alternative to traditional fluorescent, HID, and incandescent lamps. Functional advantages and benefits of LEDs include high energy conversion and optical efficiency, durability, lower operating costs, and many others. Recent advances in LED technology have provided efficient and robust lighting sources that enable a variety of lighting effects in many applications. Some of the fixtures embodying these sources feature a lighting module, including one or more LEDs capable of producing different colors, e.g. red, green, and blue, as well as a processor for independently controlling the output of the LEDs in order to generate a variety of colors and color-changing lighting effects, for example, as discussed in detail in U.S. Pat. Nos. 6,016,038 and 6,211,626, incorporated herein by reference.
Typically, in these fixtures, driver circuitry is provided to controllably supply an operating current to drive one or more LED light sources (collectively referred to as LED load). In many applications, the circuitry includes an LED driver specifically designed to supply a high load current. Such high current LED drivers are being employed extensively to drive LED loads for lighting applications.
In some cases, a short circuit may occur across the LED load. This may happen because of some error in the mechanical assembly of the LED load, a mechanical vibration of the LED assembly causing a short circuit, a defect in the LED load, water leakage into the LED load assembly, high surrounding humidity, or for other reasons. If any of these situations occur, unless additional measures are taken the LED driver will continue to try to deliver the normal full operating current to the LED load. This high current may heat the area where the short circuit exists which, in turn, may cause a variety of different hazards such fire, smoke, explosion, melting of a lighting fixture holding the LED load, or other damage to the LED load or to circuitry connecting the LED driver to the LED load.
To address this problem, it is desirable to detect any kind of short circuit or fault that occurs across the LED load. Furthermore, in the event that such a short circuit or fault occurs, it is desirable to limit the current supplied by the LED driver to the LED load to a lower, safe, value that will not cause the kinds of hazards described above.
However, providing such a current-limiting arrangement presents another problem. In many cases, the cause of the short circuit or fault is only temporary (e.g., in cases of mechanical vibration; humidity; etc.) and after a period of time the short circuit disappears or is eliminated. Nevertheless, even though the short circuit no longer exists and the LED load is capable of normal operation, because the load current supplied to the LED load by the LED driver has been reduced to a low value, the LED load can no longer operate properly.
Thus, there is a need in the art to provide a current limiting controller for a driver circuit for an LED load that can reset the driver to normal operation when a short circuit or fault is eliminated.